The trial is expected to take up to three months, meeting twice a week, according to a report by the USA -government-funded Voice of America quoting Phnom Penh Municipal Court President Taing Sunlay.
"My colleagues and I know that Kem Sokha's case is one about politicians' differing points of view, and needs to be solved through political means", he said, alluding to a past political settlement that ended a civil war with the communist Khmer Rouge.
"[Kem Sokha] wants his original version of the video to be examined, which is about one hour long".
Kem Sokha addresses the media outside his home in Phnom Penh in November 2019.
Mr Sokha's daughter Monovithya Kem called the trial against her father a "farce" as rights groups called for the case to be dismissed. It is not the guilt or innocence of Kem Sokha, but about the future political course of the Cambodian government. If found guilty, Sokha could spend 30 years in jail. A Reuters witness saw some empty seats during the Wednesday session attended by diplomats and officials.
But, she said, the video, which was the basis for the charge, could not be used to prove an attempted revolution because there were no "actual" activities to support the claim.
Kem Sokha, 66, was arrested in September 2017 on accusations of plotting to overthrow the government.
Sokha was arrested in 2017 and his party dissolved ahead of widely criticised elections the following year - leaving the CPP to canter to victory virtually uncontested.
Hun Sen's government proceeded to outlaw the opposition, taking all 125 seats in the 2018 election, making Cambodia a de-facto one party state.
Tech said Sokha's case of conspiracy with a foreign power involved nine countries, certain NGOs and foreign individuals.
"I did NOT commit anything of which I am accused - especially I haven't committed any acts that would be detrimental to the national interests", he said.
The EU began a process that could result in its withdrawal of preferential duty-free and quota-free status for imports from Cambodia because of deficiencies in labor and human rights.
"My political activities were focused on the participation in free, fair, and just elections that truly reflect the will of the Cambodian people".
Just days before the start of the trial, the court announced that only a small group of people would be allowed inside the courtroom, meaning that most observers - including most journalists - had to wait outside. "I want justice to come swiftly".
Many in the region have spoken out against the trial.
"All democrats must condemn Hun Sen to show that the charges against Kem Sokha are groundless", he wrote from self-imposed exile in Paris, France, where he has lived in 2015 to avoid a string of charges and convictions he says are politically motivated. Kem Sokha is the figurehead of democracy in Cambodia. "That's why we have to support him".
The case is among the chief concerns behind the EU's review of the country's EBA eligibility over what it describes as "severe" human rights violations.
"It is essential that human rights monitors and journalists are given unhindered access to the trial". Phil Robertson, deputy Asia director of New York-based Human Rights Watch, said it was unlikely Kem Sokha would receive a fair trial.
A Cambodian court opened a trial against the prominent Cambodian opposition leader Kem Sokha, accusing him of "conspiring with foreign powers" to overthrow the government. Written in English by Joshua Lipes. Translated by Samean Yun.
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